Primary explosives are sensitive explosive materials that are used, in relatively small quantities, to initiate a secondary or main explosive charge. Primary explosives are commonly used in percussion primers and electric primers (hot-wire igniters) to initiate an explosion. Conventionally, these primer compositions were based on lead-containing components, such as lead styphnate. The use of lead-containing material is undesirable from an environmental standpoint since its use and manufacture can contribute to or cause lead contamination.
In response, a “green” alternative to lead-containing primer compositions was developed, in which the lead styphnate or similar material is replaced with a lead-free material, One such example of a lead-free replacement for lead styphnate is 4,6-dinitro-7-hydroxybenzofuroxan, potassium salt (“KDNP”), the details of which are described in U.S. Publication No. 2009/0223401.
During the development of KDNP for replacement of lead styphnate, it became apparent that 3-aminopicric acid, also known as 3-ANP or 3-amino-2,4,6-trinitrophenol, would be a suitable synthetic intermediate. Prior art teaches that 3-ANP may be formed by reaction of water on 2,3,4,6-tetranitroaniline. However, 2,3,4,6-tetranitroaniline is difficult to prepare and exists as an unstable explosive material.
An alternative starting material to 2,3,4,6-tetranitroaniline is picric acid. Picric acid is a desirable starting material to form 3-ANP because commercial picric acid is readily available and is typically less expensive than 2,3,4,6-tetranitroaniline. Moreover, a possible source of picric acid includes Explosive D (also known as Dunnite or ammonium picrate). Explosive D is stockpiled in large quantities for military purposes and some amount of this material may be scheduled for decommissioning. To avoid decommissioning costs, Explosive D could potentially be converted to picric acid and subsequently to 3-ANP. Use of Explosive D as a source of picric acid would avoid any initial nitration process and resulting waste disposal costs to afford a truly “green” synthetic methodology.
Conversion of picric acid to 3-ANP requires a reaction process to substitute an amino group in place of a hydrogen. A reaction methodology known as vicarious nucleophilic substitution (“VNS”) is commonly used in energetics chemistry to replace a hydrogen with an amino group on an electrophilic (highly nitrated) aromatic ring, particularly in the preparation of DATB, TATB, and related materials. See M. Makosza and J. Winiarski, Acc. Chem. Res. 1987, 20, 282; U.S. Pat. No. 6,069,277. The general experimental protocol for VNS involves reacting nitroarenes or sufficiently activated aromatics with hydroxylamine in either aqueous or organic solvent and often in the presence of a base such as KOH, NaOH or t-BuOK. See A. R. Katritzky and K. S. Laurenzo, J. Org. Chem., 1986, 51 (25), 5039-5040. 1,1,1-Trimethylhydrazinium iodide has also been proven to be a highly reactive reagent for aminations of nitro-aromatic compounds. See P. F. Pagoria, A. R. Mitchell, R. D. Schmidt, J. Org. Chem., 1996, 61, 2934-2935.
Converting picric acid (either commercial picric acid or picric acid derived from Explosive D) to 3-ANP in an aqueous system via the standard VNS experimental protocol is problematic. The aqueous solvents did not perform well because of the highly insoluble nature of the sodium or potassium picrate salts. Potassium picrate is extremely insoluble in water, the sodium analog is only very slightly soluble, and the solubilities of both compounds are not improved by addition of methanol. As a result, aminations of picric acid utilizing hydroxylamine/base give extremely low yields of 3-ANP and are unsuitable from a synthetic standpoint.
Due to the desire to have an inexpensive and green process for producing 3-ANP from picric acid, it is desirable to find a method to produce 3-ANP from picric acid in an aqueous system, particularly in situations where large amounts of solvent or large scale reactions are required.